Bipolar power transistors used in integrated circuits conventionally have an elongate structure. An elongate structure minimizes the area of the integrated circuit required for a transistor having a given current handling capability. The structure of a typical bipolar power transistor 10 is shown in FIG. 1. A primary consequence of the elongate structure is that the collector region 11, the collector metal 12, the emitter region 13, the emitter metal 14, the base region 15, the base metal 16, and the base-emitter junction 18 between the base region and the emitter region are also elongate. Additionally, the base-emitter junction is serrated to increase the junction length within the overall dimensions of the transistor.
The emitter metal 14 connects the emitter region 13 of the transistor to a hypothetical emitter terminal 20, and thence to other elements (not shown) of the integrated circuit. In the example shown, the emitter metal 14 is a strip of metal deposited on the surface of the emitter region and extending from the emitter region to other pans (not shown) of the integrated circuit.
The emitter metal 14 has an appreciable resistance along its length between the emitter terminal 20 and its remote end 22, remote from the emitter terminal. As a result of this resistance, the voltage VEA applied to the emitter region 13, and, hence, to the emitter side 17 of the base-emitter junction 18 differs from the voltage VET at the emitter terminal. This voltage difference progressively increases along the length of the emitter metal 14 towards the remote end 22. With the emitter current IE flowing in the direction shown in FIG. 1, the voltage applied to the emitter side 17 of the base-emitter junction progressively increases from a minimum VEP at the end of the emitter region adjacent the emitter terminal 20 to a maximum VER at the end of the emitter region adjacent the remote end 22 of the emitter metal.
The current through the base-emitter junction 18 depends on the base-emitter voltage VBE according to exp(qVBE/kT), where q is the electronic charge, VBE is the voltage between the base side 19 and the emitter side 17 of the base-emitter junction 18, k is Boltzman's constant, and T is the temperature in degrees K. Hence, the variation in the voltage applied to the emitter region 13 by the emitter metal 14 at different points along the length of the base-emitter junction causes the current through the base-emitter junction to vary significantly from one end of the base-emitter junction to the other.
For example, at an emitter current IE of 500 mA, the measured difference between the voltage at the emitter terminal VET and the voltage VER at the remote end 22 of the emitter metal was 326 mV. As a result of the corresponding difference in the voltage at the emitter side 17 of the base-emitter junction, the emitter current IER at the end of the emitter region adjacent the remote end 22 of the emitter metal was less than one one-millionth of the emitter current IEP at the end of the emitter region adjacent the emitter terminal 20. This imbalance in the emitter current resulting from the change in the voltage at the emitter side of the base-emitter junction due to the ohmic drop in the emitter metal 14 requires that the transistor be made larger than would otherwise be required to provide a given current handling capacity.
U.S. Pat. No. 4,072,979 discloses the structure shown in FIG. 2. This structure provides a substantially uniform voltage at the emitter side 37 of the base-emitter junction 38 along the length of the base-emitter junction. In the structure shown in FIG. 2, the emitter region 32 is divided into a junction zone 41, and plural contact zones 43 and plural interconnecting zones 45 arrayed along the length of the base-emitter junction 38. Each interconnecting zone 45 connects one contact zone 43 to part of the junction zone 41. The emitter metal 34 runs along the length of the emitter region 32 and contacts each contact zone 43.
Between each pair of adjacent interconnecting zones 45 is a low conductivity zone 47. The low-conductivity zone is a zone of intrinsic material, or base region material. The conductivity of the low-conductivity zone is substantially less titan that of the emitter region, so that the emitter current flows from each contact zone 43 to the junction zone 41 via the respective interconnecting zone 45. The interconnecting zones each have a resistance determined by their widths W, which differ along the length of the emitter region 32.
The resistance of each interconnecting zone 45 introduces an additional voltage difference between the emitter metal 34 and the part of the junction zone 41 adjacent the interconnecting zone. The total difference between the voltage VET at the hypothetical emitter terminal 40 and the voltage at the emitter side 37 of the base-emitter junction adjacent each interconnecting zone 45 is therefore the sum of the voltage drop in the emitter metal 34 between the emitter terminal 40 and the respective contact zone 43 and the voltage drop due to the resistance of the interconnecting zone.
To distribute the voltage at the emitter side 37 of the base-emitter junction 38 evenly along the length of the base-emitter junction, and, hence, to distribute the emitter current evenly, the width W of each interconnecting zone 45 is designed to provide a resistance that causes the voltage drop across the interconnecting zone to be such that the total voltage drop between the emitter terminal 40 and the part of the emitter side 37 of the base-emitter junction adjacent the interconnecting zone is the same for all the interconnecting zones. The width WP of an interconnecting zone 45 closer to the emitter terminal 40 is made less (giving a higher resistance) than the width WR of an interconnecting zone more remote from the emitter terminal. In this way, the voltage at the emitter side 37 of the base-emitter junction 38 is made the same along the: entire length of the base-emitter junction. This provides a significant improvement in the evenness of the distribution of the emitter current along the length of the base-emitter junction.
Providing the same voltage at the emitter side 17 of the base-emitter junction 18 at all points along the length of the base-emitter junction shown in FIG. 1 does not result in an entirely uniform emitter current distribution, however. This is because there is a secondary effect, namely, the ohmic loss of base voltage along the length of the base metal 16 caused the flow of the base current through the base metal. This causes the voltage applied to the base side 19 of the base-emitter junction to change along the length of the bate-emitter junction.
The base connection 56 connects the base region 15 of the transistor to the hypothetical base terminal 50, and thence to other elements (not shown) of the integrated circuit. In the example shown, the base connection 56 includes the base metal 16 deposited on the surface of the base plug 54 formed in the base region 15. The base plug is a highly-doped region of the same conductivity as the base region. For example, in an npn transistor, the base plug is a p+ region. In known power transistors, the base plug 54 is elongate and runs parallel to the base-emitter junction 18.
The base metal 16 has an appreciable resistance along its length between the base terminal 50 and the remote end 52 of the base metal. As a result of this resistance, the voltage VBA applied to the base region, and hence to the base side 19 of the base-emitter junction 18, differs from the voltage VBT applied to the base terminal 50. Due to the flow of the base current though the base metal, the difference between the voltage VBT at the base terminal 50 and the voltage VBA applied to the base region 15 increases along the length of the base metal towards the remote end 52. With the base current IB flowing in the base metal in the direction shown in FIG. 1, the voltage applied to the base side 19 of the base-emitter junction progressively decreases from a maximum VBP at the end of the base region adjacent the base terminal 50 to a minimum VBR at the end of the base region adjacent the remote end 52 of the base metal.
If the emitter structure shown in FIG. 2 is used to make the voltage at the emitter side 17 of the base-emitter junction 18 the same at all points along the length of the base-emitter junction, the above-described variation in the voltage at the base side 19 of the base-emitter junction 18 along the length of the base-emitter junction will reduce the emitter current carried by the part of the emitter region 12 remote from the base terminal 50. The same law of current flow discussed above applies. For example, with a base current of 50 mA, a voltage difference of 79 mV was measured on the base metal between the base terminal 50 and the remote end 52. As a result of the corresponding difference in voltage between the opposite ends of the base side 19 of the base-emitter junction 18, the emitter current carried by the pan of the emitter region 13 remote from the base terminal 50 is less than one-tenth of that carried by the pan of the emitter region adjacent the base terminal.
To mitigate the effects of ohmic loss in the base metal on current distribution, it is known to use a very wide base metal to minimize the ohmic loss in the base metal. It is also known to provide a wide region of base material between the base metal and the base-emitter junction. The voltage drop due to the resistance of the wide region of base material effectively swamps the change in voltage between the opposite ends of the base metal 16. Both of these solutions, however, have the effect of increasing the area required to provide a transistor having a given current handling capability.
Another way of providing a more even current distribution in an integrated circuit power transistor is to use a power transistor with a radial structure. In a radial structure, the base-emitter junction is centered on the emitter terminal. This structure makes the path length between the emitter terminal and all points on the emitter side of the base-emitter junction the same. This structure, however, requires two metallization layers because the base terminal must overlay the emitter terminal to enable all points on the base side of the base-emitter junction also to be equidistant from the base terminal. To balance the current distribution in the transistor in this manner requires more complicated processing because of the second metallization layer, and results in a structure that uses the available area of the integrated circuit less efficiently.